Radiographic differentiation between cystic fibrosis and asthma presenting in teenagers and young adults can be difficult. Many patients with a late presentation of cystic fibrosis display minimal changes on a chest radiograph. However, a large majority (90%) of cystic fibrosis patients with an essentially normal PA chest radiograph will have a distinctly outlined orifice of right upper lobe bronchus on a lateral chest film as opposed to a small number of asthmatics (25%) or normal patients (18%). This observation correlates well with the pathologic finding that the initial pulmonary involvement in cystic fibrosis is typically in the right upper lobe in adolescents. Teenage or young adult patients presenting with a history of repeated respiratory infections, asthma-like symptoms and a distinctly visible right upper lobe bronchus on a lateral chest film should be sweat-tested to exclude cystic fibrosis.